Optical transmission systems for telecommunications typically comprise a number of system elements connected by waveguides in the form of optical fibres, there being typically a number of bi-directional line amplifiers at spaced locations between each transmitter and receiver. Such amplifiers are necessary when long distance communication is required in order to compensate for the power loss associated with transmission through the fibres.
One of the main factors limiting the maximum power which can be launched into a fibre is the effect of stimulated Brillouin scattering, the effects of which are characterised by a counterpropagated signal which is down shifted in frequency by about 11 GHz for silica glass fibres, the onset of scattering being observed at a threshold power which can be as low as 2 mW to 3 mW.
SBS (stimulated Brillouin scattering) can be suppressed by low frequency amplitude modulation (dither) of the transmission as described in U.S. Pat. No. 5,329,396 so that the peak launched power level can be increased. In effect, this modulation enables the power spectral density of the transmission to be less than the threshold of onset of SBS. A similar effect is achieved by time varying the phase angle of transmission light waves as disclosed in U.S. Pat. No. 4,560,246. There is however some degree of degradation to the transmitted signal associated with such methods of SBS suppression so that the degree of suppression utilised is ideally kept to a minimum.
Other forms of induced counterpropagating process include Rayleigh scattering and Raman scattering. In referring to induced counterpropagating processes in the context of the present application, the intended meaning encompasses the above mentioned and similar effects associated with wavelength dependent and/or non linear sources of counterpropagating signal, generally associated with bulk properties of the waveguide material, and specifically excluding reflection from fibre defects, couplings or other discontinuities in the waveguides or components of the system.
U.S. Pat. No. 4,997,277 includes a discussion of the detection of Rayleigh scattering as a means of investigating the distributed properties of an optical fibre and the use of Brillouin light amplification to investigate optical fibre properties.
It is also known from U.S. Pat. No. 5,513,029 to provide an optical transmission system in which an optical signal is modulated with a low frequency dither signal enabling individual signal and noise components in a wavelength multiplexed system to be measured at monitoring locations in the system and wherein optical amplifiers of the system are controlled so as to be responsive to the monitored data to control the optical gain profile of the optical amplifier.